Gas-fueled, compression ignition engine with maximized pilot ignition intensity

ABSTRACT

Pilot fuel injection and/or ignition are controlled in a pilot ignited, gas-fueled, compression ignition engine so as to maintain a relationship Dp/Di of &lt;1, where Dp is the duration of the pilot injection event and Di is the injection delay period as measured from the start of initiation of pilot fuel injection (Tp) to the start of pilot fuel autoignition (Ti). Dp/Di is less than 1 when a mixing period Dm exists between the end of pilot fuel injection and the start of autoignition. This mixing period permits the injected pilot fuel to become thoroughly distributed through and mixed with the gaseous fuel/air charge in the combustion chamber and vaporized prior to ignition, resulting in improved premixed burning of a heterogeneous mixture of the pilot fuel, the gaseous fuel, and air and dramatically reduced NO X  emissions. Dp/Di (or a characteristic of it such as Di or Dm) preferably is maintained within a predetermined range on a cycle-by-cycle, full speed and load range basis so as to maximize ignition intensity under all engine operating condition. The resultant maximization of pilot ignition intensity can generate instantaneous power on the order of 200 kW/l of engine displacement.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to engines powered atleast partially by a gaseous fuel such as natural gas (hereafterdescribed as “gas-fueled engines”) and, more specifically, to agas-fueled engine which is of the compression ignition type and whichincorporates measures to inject a pilot fuel into a combustion chamberof the engine during its compression stroke, thereby permitting ignitionof the gaseous fuel charge by compression ignition. The inventionadditionally relates to a method for maximizing the pilot fuel ignitionintensity in a gas-fueled, compression ignition engine.

[0003] 2. Discussion of the Related Art

[0004] Recent years have seen an increased demand for the use of gaseousfuels as a primary fuel source in compression ignition engines. Gaseousfuels such as propane or natural gas are considered by many to besuperior to diesel fuel and the like because gaseous fuels are generallyless expensive, and when used in compression ignition engines, provideequal or greater power with equal or better fuel economy, and producesignificantly lower emissions. This last benefit renders gaseous fuelsparticularly attractive because recently enacted and pending worldwideregulations may tend to prohibit the use of diesel fuel as the primaryfuel source in many engines. The attractiveness of gaseous fuels isfurther enhanced by the fact that existing compression ignition enginedesigns can be readily adapted to burn these gaseous fuels.

[0005] One drawback of gaseous fuels is that they exhibit significantlyhigher ignition threshold temperatures than do diesel fuel, lubricatingoil, and other liquid fuels traditionally used in compression ignitionengines. The compression temperature of the gas and air mixture isinsufficient during operation of standard compression ignition enginesfor autoignition. This problem can be overcome by igniting the gaseousfuel with a spark plug or the like. It can also be overcome by injectinglimited quantities of a pilot fuel, typically diesel fuel, into eachcombustion chamber of the engine in the presence of a homogenous gaseousfuel/air mixture. The pilot fuel ignites after injection and burns at ahigh enough temperature to ignite the gaseous fuel charge.Pilot-ignited, compression ignition, gas-fueled engines are sometimescalled “dual fuel” engines, particularly if they are configured to runeither on diesel fuel alone or on a combination of diesel fuel and agaseous fuel. They are often sometimes referred to as MicroPilot®engines (MicroPilot is a registered trademark of Clean Air Partners,Inc. of San Diego, Calif.), particularly if the pilot fuel injectors aretoo small to permit the use of the engine in diesel-only mode. Thetypical true “dual fuel” engine uses a pilot charge of 6 to 10% ofmaximum fuel rate. This percentage of pilot fuel can be reduced to 1% ofmaximum, or even less, in a MicroPilot® engine. The invention isapplication to true dual fuel engines, MicroPilot® engines, and otherpilot-ignited, compression ignition, gas-fueled engines as well. It willbe referred to simply as a “dual fuel engine” for the sake ofconvenience.

[0006] A disadvantage of dual fuel engines over spark-ignited engines isthe potential generation of increased quantities of oxides of Nitrogen(NO_(X)) resulting from sub-maximum ignition intensity of the pilot fuelcharge and resultant less than optimal combustion of the pilot and gasfuel charges. The inventors theorize that less than maximum ignitionintensity results from failing to time pilot fuel autoignition to atleast generally occur after optimal penetration, distribution, andvaporization of the pilot fuel charge in the gas/air mixture. Ifautoignition (defined as the timing of initiation of pilot fuelcombustion) occurs too soon after pilot fuel injection, the pilot fuelwill be heavily concentrated near the injector because it has not yettime to spread throughout the combustion chamber. As a result, overlyrich air/fuel mixtures are combusted near the injector, while overlylean mixtures are combusted away from the injector. Conversely, ifautoignition occurs too long after pilot fuel injection, excessive pilotfuel vaporization will occur, resulting in misfire.

[0007] Moreover, premixed combustion of the pilot fuel, i.e., combustionoccurring after the fuel mixes with air, provides greater ignitionintensity than diffusion combustion, i.e., combustion occurringimmediately upon injection into the combustion chamber and before thefuel mixes with air. Maximizing premixed combustion of pilot fuel isenhanced by retarding autoignition to give the pilot fuel an opportunityto thoroughly mix with the air and form a homogeneous gas/pilot/airmixture. However, retarding autoignition timing is usually consideredundesirable in diesel engine technology. In fact, it is almostuniversally agreed that optimum combustion in a conventional compressionignition diesel engine is achieved with the shortest possible ignitiondelay, and it is generally preferred that the ignition delay periodshould always be much shorter than the injection duration in order avoidan excessive rate of pressure rise, high peak pressure, and excessiveNO_(X) emissions. (See, e.g., SAE, Paper No. 870344, Factors That AffectBSFC and Emissions for Diesel Engines: Part II Experimental Confirmationof Concepts Presented in Part I, page 15). Conventional dual fuelengines, however, do not allow sufficient mixing time to maximizeignition intensity by igniting a pilot charge that is largely pre-mixed.

[0008] The need has therefore arisen to maximize the ignition intensityof a dual fuel charge.

SUMMARY OF THE INVENTION

[0009] It has been discovered that the relationship between ignitiondelay and injection duration is an important consideration when pilotinjection is optimized for achieving the most intense ignition. The bestperformance is achieved when the fuel and combustion environment arecontrolled such that the duration of injection of pilot fuel is lessthan the ignition delay period (defined as the time between start ofpilot fuel injection and the start of pilot fuel autoignition). Statedanother way, the best performance is obtained when the ratio Dp/Di<1,where Dp is the injection period and Di is the ignition delay period. Itis believed that the pilot spray becomes thoroughly pre-mixed during themixing period Dm occurring between the end of pilot fuel injection andthe beginning of autoignition, Ti. This thorough premixing leads tomaximized ignition intensity and dramatically reduced emissions. Hence,the inventors have surprisingly discovered that improved results stemfrom proceeding directly away from the conventional wisdom of providingan ignition delay period that is shorter than the injection durationperiod. However, in the preferred embodiment, the mixing period Dmpreferably should be controlled to also be sufficiently short to avoidmisfire.

[0010] The ratio Dp/Di can be varied by varying pilot fuel injectiontiming, pilot fuel injection duration, or autoignition timing. BecauseDp/Di is dependent on ignition delay, the ratio Dp/Di can be optimizedfor a given Di by determining an optimum Dm and adjusting engineoperating parameter(s) as necessary to obtain the determined optimum Dm.This control is preferably performed on a full time, full speed and loadrange basis. It may be either open loop or closed loop.

[0011] Ignition intensity maximization can also be thought of in termsof the peak power generated by the pilot ignition. If injection andautoignition are controlled to maximize the number and distribution ofpilot fuel droplets and to minimize their size, ignition power on theorder of 100 kW/l is obtainable, resulting in extremely effectiveignition of the gaseous fuel charge. Ignition under these circumstancescan be considered analogous to the simultaneous energization of tens ofthousands of tiny spark plugs distributed throughout the gas/fuelmixture.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Preferred exemplary embodiments of the invention are illustratedin the accompanying drawings, in which like reference numerals representlike parts throughout and in which:

[0013]FIG. 1 schematically illustrates the fuel supply systems of aninternal combustion engine on which the inventive ignition intensitymaximization control scheme can be implemented;

[0014]FIG. 2 schematically illustrates the combustion airflow controlsystems of the engine of FIG. 1;

[0015]FIG. 3 is a partially schematic sectional side elevation view of aportion of the engine of FIGS. 1 and 2;

[0016]FIG. 4 is a somewhat schematic, partially sectional, sideelevation view of a pilot fuel injector assembly usable in the engine ofFIGS. 1-3 and showing the injector in its closed position;

[0017]FIG. 5 corresponds to FIG. 5 but shows the injector in its openposition;

[0018]FIG. 5a is an enlarged view of a portion of a nozzle of the fuelinjector assembly of FIG. 5;

[0019]FIG. 6 graphically illustrates the dispensing of a jet spray froman ECIS-type fuel injector;

[0020]FIG. 7 is a graph of velocity versus needle lift at the bottom ofthe discharge passage for both a bottom seated pintle nozzle and a topseated pintle nozzle;

[0021]FIG. 8 schematically represents an electronic controller for theengine of FIGS. 1-3;

[0022]FIG. 9 is graph illustrating the effect of changes in ignitiondelay on NO_(X) emissions under a particular set of engine operatingconditions;

[0023]FIG. 10 is a set of graphs illustrating fuelpenetration/distribution percentage and spray vaporization percentagevs. mixing period for various air charge temperatures (ACTs);

[0024]FIG. 11 is a set of graphs illustrating combustion characteristicsof a dual fuel engine;

[0025]FIG. 12 is a set of graphs illustrating the effects of varying ACTon ignition delay at various pilot fuel injection timings;

[0026]FIG. 13 is a set of graphs illustrating the effects of ignitiondelay on mixing times at various Dp/Di ratios;

[0027]FIG. 14 is a flowchart illustrating an open loop control schemefor maximizing pilot fuel ignition intensity in accordance with theinvention; and

[0028]FIG. 15 is a flowchart illustrating a closed loop control schemefor maximizing pilot fuel ignition intensity in accordance with theinvention.

[0029] Before explaining embodiments of the invention in detail, it isto be understood that the invention is not limited in its application tothe details of construction and the arrangement of the components setforth in the following description or illustrated in the drawings. Theinvention is capable of other embodiments or being practiced or carriedout in various ways. Also, it is to be understood that the phraseologyand terminology employed herein is for the purpose of description andshould not be regarded as limiting.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] 1. Resume

[0031] Pursuant to the invention, pilot fuel injection and/or ignitionare controlled in a pilot ignited, gas-fueled, compression ignitionengine so as to maintain a relationship Dp/Di of <1, where Dp is theduration of the pilot fuel injection event and Di is the injection delayperiod, as measured from the start of initiation of pilot fuel injection(Tp) to the start of pilot fuel autoignition (Ti). Although this controlproceeds contrary to conventional wisdom, the inventors have discoveredthat the mixing period (Dm) resulting from maintaining an ignition delayperiod that is longer than an injection period maximizes ignitionintensity by permitting the injected fuel to become thoroughlydistributed through and mixed with the gaseous fuel/air charge in thecombustion chamber prior to ignition. This, in turn, results in improvedpremixed burning of a nearly homogeneous mixture of the pilot fuel, thegaseous fuel, and air and dramatically reduced NO_(X) emissions.

[0032] In practice, the ratio Dp/Di (or a characteristic of it such asDi or Dm) preferably is maintained within a predetermined range on acycle-by-cycle, full speed and load range basis so as to maximizeignition intensity under all engine operating conditions. Dp/Di can bestbe optimized by adjusting Tp, Ti, or a combination of both. This controlmay be either open loop, using look-up tables or the like, or closedloop, using an ignition intensity-dependent parameter for feedback. Theresultant maximization of pilot ignition intensity can generateinstantaneous power of pilot ignition on the order of 200 kW/l of enginedisplacement.

[0033] 2. System Overview

[0034] a. Basic Engine Design

[0035] Turning now to the drawings and initially to FIGS. 1-3 inparticular, an engine on which the invention can be implemented isillustrated. Engine 10 is a dual fuel engine having a plurality ofcylinders 12 each capped with a cylinder head 14 (FIG. 3). As is alsoshown in FIG. 3, a piston 16 is slidably disposed in the bore of eachcylinder 12 to define a combustion chamber 18 between the cylinder head14 and the piston 16. Piston 16 is also connected to a crankshaft 20 ina conventional manner. Conventional inlet and exhaust valves 22 and 24are provided at the end of respective passages 26 and 28 in the cylinderhead 14 and are actuated by a standard cam shaft 30 so as to control thesupply of an air/fuel mixture to and the exhaust of combustion productsfrom the combustion chamber 18. Gases are supplied to and exhausted fromengine 10 via an intake air manifold 34 and an exhaust manifold 35,respectively. However, unlike in conventional spark ignited gas fueledengines, a throttle valve which would normally be present in the intakemanifold 34 is absent or at least disabled, thereby producing an“unthrottled” engine. An intake air control system may also be providedfor reasons detailed below.

[0036] b. Air and Fuel Delivery Systems

[0037] Gaseous fuel (e.g., compressed natural gas (CNG), liquefiednatural gas (LNG) or propane) could be supplied via a single meteringvalve discharging into a mixing body at the entrance of the manifold 34,or via a similarly-situated mechanically controlled valve. In theillustrated embodiment, however, a separate injector 40 is provided foreach cylinder 12. Each injector 40 receives natural gas, propane, oranother gaseous fuel from a common tank 39 and a manifold 36 and injectsfuel directly into the inlet port 26 of the associated cylinder 12 via aline 41.

[0038] The engine 10 is supplied with pilot fuel with multipleelectronically controlled liquid fuel injector assemblies 32. Each pilotfuel injector assembly 32 could comprise any electronically controlledinjector and an associated equipment. Examples of suitable injectors are(1) a pressure-intensified accumulator-type hydraulic electronic unitinjector of the type disclosed in U.S. Reissue Pat. No. 33,270 and U.S.Pat. No. 5,392,745, and (2) a pressure-intensified non-accumulator typehydraulic electronic fuel injector of the type disclosed in U.S. Pat.No. 5,191,867, the disclosures of all of which are hereby incorporatedby reference in their entirety, or a high pressure common rail system.The preferred injector assembly is a so-called OSKA-ECIS injectorassembly, described below.

[0039] Referring to FIGS. 1 and 3, injector assembly 32 is fed with fuelfrom a conventional tank 42 via a supply line or common rail 44.Disposed in line 44 are a filter 46, a pump 48, a high pressure reliefvalve 50, and a pressure regulator 52. A return line 54 also leads fromthe injector 32 to the tank 42. The fuel may be any fuel suitable foruse in a compression-ignition engine. Diesel fuel is most commonly usedfor pilot fuel in dual fuel engines of the disclosed type. However,engine lubricating oil may also be used. Engine lubricating oil isparticularly attractive in MicroPilot® applications because thoseapplications require such small quantities of pilot fuel (typicallycomprising, on average, no more than about 1% of the total fuel chargesupplied to the combustion chamber) that the lubricating oil can bereplenished continuously, keeping the oil fresh and obviating the needfor oil changes.

[0040] Gaseous fuel could be supplied via a single metering valvedischarging into a single throttle body at the entrance of the manifold34, or via a similarly-situated mechanically controlled valve. In theillustrated embodiment, however, a separate injector 40 is provided foreach cylinder 12. Each injector 40 receives natural gas, propane, oranother gaseous fuel from a common tank 39 and a manifold 36 and injectsfuel directly into the inlet port 26 of the associated cylinder 12 via aline 41.

[0041] Referring to FIG. 2, the air intake control system may include(1) an exhaust gas recirculation (EGR) subsystem permitting recirculatedexhaust gases (EGR) to flow from an exhaust manifold 35 to the intakemanifold 34 and/or filtered for removal of soot, (2) a turbochargingsubsystem which charges non-EGR air admitted to the intake manifold 34.The EGR subsystem, which changes EGR and airflow, is useful forincreasing ignition delay, diluting the charge, reducing the peakcombustion temperature, and inhibiting the formation of NO_(X)emissions. It includes (1) an EGR cooler 59 and an EGR metering valve 60located in a return line 58 leading from the exhaust manifold 35 to theintake manifold 34. The line 58 may be connected to the exhaust linecontaining the wastegate 74 (detailed below) at its inlet end, andpreferably empties into the air intake line at its outlet end with theaid of a mixing venturi 61. An EGR filter 63 is also located in the line58, upstream of the EGR cooler, to reduce diesel soot. A second line 62leads from a turbo bypass valve 76 and back to the air inlet system. Inaddition, an exhaust back pressure (EBP) valve 68 having an adjustableflow-restricting metering orifice may be provided in the exhaust gasstream to control the exhaust gas absolute pressure (EGAP), hencevarying EGR flow. Valve 68, if present, can be actuated by a controller56 (FIG. 6) to adjust the percentage of EGR in the total charge admittedto intake port 66 without controlling valve 60.

[0042] As is further shown in FIG. 2, the turbocharging subsystem of theintake air control system includes a turbocharger 70 and an aftercooler72 provided in line 62 upstream of the valve 60 and intake port 66.Operation of the turbocharger 70 is controlled in a conventional mannerby a wastegate 74 and a turbo bypass 76, both of which areelectronically coupled to the controller 56 (detailed below). Otherintake airflow modification devices, such as a supercharger, a turbo-airbypass valve, or EGR modification devices, such as an expansion turbineor an aftercooler, may be employed as well. Examples of ways in whichthese devices may be operated to adjust engine operating parameters suchas air charge temperature (ACT), excess air ratio (lambda), and manifoldabsolute pressure are provided in co-pending and commonly assigned U.S.patent application Ser. No. 08/991,413 (the '413 application) andentitled Optimum Lambda Control for Compression Ignition Engines, filedin the name of Beck et al. The disclosure of the '413 application isincorporated by reference by way of background information.

[0043] c. OSKA-ECIS Fuel Injector Assembly

[0044] The OSKA-ECIS fuel injector assembly 32 utilized in the preferredand illustrated embodiment of the invention, comprises 1) a highdischarge coefficient injector 300, 2) a so-called OSKA infringementtarget 302, and 3) a toroidal chamber 304 located in a cavity in theupper surface 306 of the piston 16. The injector 300 discharges ahigh-velocity stream at a rapidly falling rate so as to provide anExpanding Cloud Injection Spray (ECIS). The injected stream of fuelimpinges against the target 302, which breaks the fuel droplets intosmaller droplets and reflects the fuel into the chamber 304 as adispersed, vaporized spray. The spray then swirls through the chamber304 in a highly turbulent manner so as to maximize the rate ofpenetration, distribution, vaporization, and mixing with the air/fuelmixture in the chamber 18.

[0045] The injector 300 is preferably an accumulator type injector suchas the ones described, e.g., in U.S. Reissue Pat. No. 33,270 and in U.S.Pat. No. 5,392,745, the disclosures of both of which are incorporated byreference. In an accumulator type fuel injector, the injection pressurefalls from an initial peak as a square function, and the injectionvelocity falls as a square root function of pressure. Hence, thevelocity falls essentially as a straight-line function during theinjection event. Stated another way, because all or nearly all of thepilot mass is injected at a uniformly falling rate, each successive massof droplets ejected from the nozzle moves slower than the mass beforeit, and the droplets therefore do not have the opportunity toaccumulate. This effect is illustrated in the diagram of FIG. 6, whichshows the separation resulting from a rapidly falling injection velocityor -dUj/dt.

[0046] Also as discussed in the '745 patent, the ECIS effect can beenhanced by utilizing a nozzle in the injector that has a relativelyhigh discharge coefficient when compared, e.g., to a conventionalvalve-covers-orifice (VCO) nozzle. A hollow nozzle having a single,relatively large discharge orifice pointed directly at the target 302would suffice. The preferred nozzle 310, however, is a so-calledbottom-seated pintle nozzle of the type described, e.g., in U.S. Pat.No. 5,853,124, the subject matter of which is incorporated by reference.In that type of nozzle, a negative interference angle is formed betweena conical tip of the needle and the mating conical valve seats so thatthe needle seat is located at the bottom of the valve seat rather thanat the top. The resulting nozzle lacks any velocity drop downstream ofthe needle seat, even at very low needle lifts, so that virtually all ofthe energy used to pressurize the fuel is converted to kinetic energy.Spray dispersion and penetration at low needle lifts therefore aresignificantly enhanced.

[0047] Referring to FIGS. 3-5 a, the pintle nozzle 310 includes a nozzlebody 312 in which is housed a needle valve assembly that includes anozzle needle 314 and a valve seat 316. The nozzle needle 314 isslidably received in a bore 318 extending axially upwardly into thenozzle body 312 from the valve seat 316. A pressure chamber 319 isformed around the lower portion of the nozzle needle 314 and is coupledto the fuel source 42 by a fuel inlet passage (not shown) and the inletline 44. The lower end of the needle 314 forms a tip 328. The upper endof the nozzle needle 314 is connected to a needle stem (not shown) thatin turn is guided by a bushing or other needle guide (also not shown)for concentric movement with the bore 318. The nozzle needle 314 isbiased downwardly towards the valve seat 316 by a return spring (alsonot shown) acting on an upper surface of the needle guide. A relativelyshort cylindrical passage 324 is formed in the nozzle body 312 beneaththe valve seat 316 and opens into a bottom surface 326 of the nozzlebody 312 for purposes detailed below.

[0048] Referring to FIG. 5a, the valve seat 316, which typically ismachined directly into the nozzle body 312 and forms the bottom endportion of the bore 318, terminates in a seat orifice 330. The needletip 328 is configured to selectively 1) seat on the valve seat 316 toprevent injection and 2) lift from the valve seat 316 to permitinjection. A discharge passage 332 is formed between the valve seat 316and the needle tip 328 when the needle tip 328 is in its lifted positionof FIGS. 5 and 5a to permit fuel to flow from the pressure chamber 319,through the discharge passage 332, and out of the injection valveassembly 32 through the seat orifice 330. The valve seat 316 and atleast a portion of the needle tip 328 that seals against the valve seat316 are generally conical or frusto-conical in shape (the term conicalas used herein encompassing structures taking the shape of a right anglecone as well as other structures that decrease in cross sectional areafrom upper to lower ends thereof).

[0049] The needle tip 328 includes a frusto-conical portion 334 forengagement with the valve seat 316 and terminates in a bottom surface336. The frusto-conical portion 334 is longer than the valve seat 316but could be considerably shorter or even could take some other shape solong as it is configured relative to the valve seat 16 to be “bottomseating” as that term is defined below. The bottom surface 336 of theneedle tip 328 remains recessed within the cylinder head 14, even whenthe injector 300 is in its closed position of FIG. 4, to protect theneedle tip 328 from the hot gases in the combustion chamber 18. In orderto produce a concentrated “laser” stream configured to impinge on thetarget 302 with maximum force, the nozzle 300 terminates in a so-calledzero degree pintle pintle, lacking any structure that extends beneaththe conical valve seat 316 when the needle tip 328 is in its closed orseated position. It has been found that, in a zero-pintle pintle, sprayfrom the zero degree pintle pintle nozzle takes the form of apencil-thin jet.

[0050] In the preferred and illustrated embodiment, the pintle nozzle300 is a so-called unthrottled pintle nozzle in which the area of thegap formed between the pintle 336 and the peripheral surface of thecylindrical passage 324 is always larger than the effective area of theseat orifice 330 so that minimum flow restriction takes place downstreamof the valve seat 326. This configuration assures that fuel isdischarged from the nozzle 300 at the maximum velocity—an importantconsideration at low needle lifts and small fuel injection quantities.

[0051] The included angle α of the valve seat cone and the includedangle β of the needle tip cone usually are different so that an includedinterference angle θ is formed therebetween in order to assure seatingat a distinct needle seat that extends only part way along the length ofthe valve seat 316 and that theoretically comprises line contact. Theinterference angle θ is set to be negative so that the conical portion334 of the needle tip 328 seats against a needle seat 342 located at thebottom end of the valve seat 316 at a location at or closely adjacent tothe seat orifice 330, hence producing a bottom seated pintle nozzle. Asa result, the cross-sectional area of the passage 332 increasescontinuously from the seat orifice 330 to its upper end. Theinterference angle θ should be set sufficiently large so that seating atthe desired location at the bottom of the valve seat 316 is achieved,but must be set sufficiently small so to distribute the impact forcesoccurring upon needle closure sufficiently to avoid undue impactstresses on the needle tip 328 and valve seat 316. Preferably, theinterference angle θ should range between 0.5° and 2°, and it mostpreferably should be set at about 1°.

[0052] In operation, the nozzle needle 314 of the nozzle 310 is normallyforced into its closed or seated position as seen in FIG. 4 by thereturn spring (not shown). When it is desired to initiate an injectionevent, fuel is admitted into the pressure chamber 319 from the fuelinlet passage 320. When the lifting forces imposed on the needle 314 bythe pressurized fuel in the pressure chamber 319 overcome the closingforces imposed by the spring and decaying fluid pressure in theaccumulator injector's control cavity, the nozzle needle 314 lifts topermit fuel to flow through the discharge passage 332, past the needleseat 342, out of the seat orifice 330, and then out of the nozzle 310.The nozzle needle 314 closes to terminate the injection event when thefuel pressure in the pressure chamber 319 decays sufficiently to causethe resulting lifting forces drop to beneath the closing force imposedon the needle 314 by the return spring.

[0053] The flow area at the top of the discharge passage of aconventional top seating pintle (TSP) is less than the area at the seatorifice for needle lift values of 0.0 to 0.035 mm. On the other hand,the flow area of the discharge passage of the bottom seating pintle(BSP) 300 is less at the seat orifice 330 than at the top of thedischarge passage 332 for all values of at needle lift. The laws ofcontinuity or flow consequently dictate that the flow velocity at theseat orifice 330 of the BSP will be less than that at the upper end ofthe discharge passage by an amount proportional to the difference inflow area at the seat orifice 330 as compared to that at the upper endof the discharge passage 332. For example, at a needle lift of 0.005 mm,the flow area at the top of the discharge passage of a TSP nozzle is0.0125 mm², and the area at the bottom of the passage is 0.025 mm², or aratio of 0.5:1.0. This difference may seem inconsequential at firstglance. However, considering that, at the same needle lift and flowrate, the flow area of the nozzle 300 is 0.045 mm² at the top of thedischarge passage 332 and 0.0125 mm at the bottom, i.e., at the seatorifice 330. The spray velocity at the outlet or seat orifice of thebottom seated pintle nozzle therefore will be twice that of the topseated pintle nozzle at the same needle lift due to the converging flowarea of the discharge passage 332 of the BSP 300. Since the kineticenergy of the spray is proportional to the square of the velocity, thespray energy of the BSP 300 will be four times that of a comparable topseated pintle at the same needle lift and volumetric flow rate. This, inturn, permits rapid mixing and vaporization of the injected fuel.

[0054] The import of this effect can be appreciated by the curves 370and 372 in FIG. 7, which plot fluid velocity at the bottom of thedischarge passage for both a BSP and a TSP. Particularly relevant arethe curves which illustrate that, at needle lifts beneath about 0.03 mm,the velocity at the bottom of the discharge passage of the BSP issubstantially higher than at the bottom of the discharge passage of theTSP. At a lift of 0.01 mm, the spray velocity of the BSP is 175 m/s vs.121 m/s for the TSP, or an energy ratio of 2:1.

[0055] The enhanced velocity provided by the bottom seated pintle nozzle300 produces a spray velocity at the seat orifice of twice that of a topseated pintle nozzle of otherwise similar configuration and operatingunder the same needle lift and injection pressure. This enhancedvelocity provides a two-fold advantage in an OSKA-ECIS fuel injector andimpingement target assembly. First, it permits the injection of agreater quantity of fuel per unit time, thereby permitting the use of ashorter Dp to inject a given volume of pilot fuel and, therefore,facilitates the achievement of Dp/Di on the order of 0.2 or less.Second, the impingement of the high velocity jet against the impingementtarget 302 maximizes spray energy and further enhances the enhancedmixing effects provided by the OSKA target 302, thereby further reducingDm.

[0056] Referring again to FIGS. 4 and 5, the OSKA target 302 isgenerally of the type disclosed in U.S. Pat. No. 5,357,924, the subjectmatter of which is incorporated herein by reference. Target 302 ismounted on a platform 350 extending upwardly from the center of thecavity 304. The target 302 preferably comprises a flat-headed insertthreaded or otherwise inserted into a bore 352 in the top of theplatform 350. The insert is hardened when compared to the remainder ofthe cast metal piston 16 to mitigate and a tendency towards erosion. Anupper surface 354 of the target 302 comprises a substantially flatcollision surface for the incoming stream of injected fuel. An annulararea 356, surrounding the target 302 and formed radially between theedge of the platform 350 and the target 302, serves as a transition areathat promotes flow of reflected fuel into the toroidal chamber 304 in amanner that enhances the swirling motion provided by the toroidal shapeof the chamber 304.

[0057] The chamber 304 is not truly toroidal because the top of thetoroid is reduced by truncating an upper surface 360 of the piston 16.This truncation (1) provides the clearance volume and compression ratiorequired for a compression ignition engine, and (2) truncates an innerperiphery 362 of the upper surface of the toroid to prevent theformation of a knife-edge, thereby rendering the piston's structure morerobust. The degree of truncation is set to cause the upper surface 360of the piston 14 to nearly contact the lowermost surface 362 of thecylinder head 16 at the piston's TDC position, thereby enhancing theso-called “squish mixing” effect that results when an air/fuel mixtureis trapped between a very small gap between the uppermost surface 360 ofthe piston 16 and the lowermost surface 364 of the cylinder head 14.

[0058] The cross-section of the chamber 304 is set to provide a volumerequired to provide the engine's rated compression ratio. In an enginehaving a 16:1 compression ratio, the torrid cross-section has a diameterD_(TORIOD) that is about 0.25X D_(BORE), where D_(BORE) is the diameterof the bore in which the piston is disposed. Hence, in the case of a 140mm diameter bore, each toroid will have a diameter of 35 mm. Theindividual toroids of the chamber 304 will have a center-to-centerspacing of 55 mm. Conversely, D_(TOROID) would equal about 0.20 D_(BORE)to obtain a 20:1 compression ratio, and about 0.30 D_(BORE) to obtain a12:1 compression ratio.

[0059] The general size and configuration of the nozzle 300, the target302, and the chamber 304 are selected to achieve the desired Dp/Direduction and Dm reduction effects while maximizing the desired ECISeffect. The ECIS effect is best achieved when the fuel is injected at avelocity in a range that falls from an initial peak velocity of about200 to 250 m/s (preferably 230 m/s) to a final velocity of about 130 to220 m/s (preferably 160 m/s). These effects are achieved by obtaininginjection pressures from 20 to 30 mPa with a cylinder pressure of 5 to10 mPa.

[0060] With these constraints in mind, it is found that the optimalinjector and spray dimensions for a piston diameter of 140 mm and apilot fuel quantity, Q_(PILOT), of 2 mm³ are approximately as follows:TABLE 1 PREFERRED OSKA-ECIS INJECTOR CHARACTERISTICS EspeciallyPerferred System Characteristic Preferred Range Value Injector seatdiameter 0.35 mm to 0.70 mm  0.5 mm Needle lift 0.05 mm to 0.15 mm  0.1mm Spray diameter 0.30 mm to 0.35 mm 0.32 mm

[0061] Because the OSKA target 302 will break up the spray to dropletsto sizes that are on the order of 5 to 10% of the incoming spraydiameter, and because the droplets will travel a distance of about 500to 1000 droplet diameters in an ECIS-type injection event, the resultantOSKA-ECIS injector assembly 32 will distribute the droplets in a spaceof 25 to 100 times the initial spray diameter or 8 to 32 mm as a firstapproximation. The resulting arrangement permits the maximization offuel penetration, distribution, and vaporization during a minimized Dm,thus greatly facilitating Dp/Di and Dm minimization and facilitating theactive control of these characteristics to optimize ignition intensity.

[0062] d. Electronic Control System

[0063] Referring to FIG. 8, the controller or electronic control unit(ECU) 56 may comprise any electronic device capable of monitoring engineoperation and of controlling the supply of fuel and air to the engine10. In the illustrated embodiment, this ECU 56 comprises a programmabledigital microprocessor. Controller or ECU 56 receives signals fromvarious sensors including a governor position or other power demandsensor 80, a fuel pressure sensor 81, an engine speed (RPM) sensor 82, acrank shaft angle sensor 84, an intake manifold absolute pressure (MAP)sensor 86, an intake manifold air charge temperature (ACT) sensor 88, anengine coolant temperature sensor 90, a sensor 92 measuring exhaust backpressure (EBP), and a sensor 94 monitoring the operation of thewastegate 74, respectively. The controller 56 also ascertains EGAPeither directly from an EGAP sensor 98, or indirectly from the EBPsensor 92 (if EBP valve 68 is used). Other sensors used to control fuelinjection are illustrated at 100 in FIG. 8. Other values, such asindicated mean effective pressure (IMEP) and the mass and quantity ofgas (Q_(GAS) and V_(GAS), respectively) injected are calculated by thecontroller 56 using data from one or more of the sensors 80-100 andknown mathematical relationships. Still other values, such as intakemanifold absolute pressure (MAP), indicated mean effective pressure(IMEP), maximum engine speed (RPM), volumetric efficiency fuel quality,and various system constants are preferably stored in a ROM or otherstorage device of controller 56. Controller 56 manipulates these signalsand transmits output signals for controlling the diesel rail pressureregulator 52, the pilot fuel injector assemblies 32, and the gasinjectors 40, respectively. Similar signals are used to control theturbo wastegate 74, the turbo bypass 76, and the metering orifice or EBPvalve 68, respectively.

[0064] 3. Ignition Intensity Maximization

[0065] a. Ignition Intensity Maximization Through Dp/Di Control

[0066] i. Basic Theory

[0067] Pursuant to a preferred embodiment of the invention, thecontroller 56 (1) receives the signals from the various sensors, (2)performs calculations based upon these signals to determine injectionand/or combustion characteristics that maximize ignition intensity, and(3) adjusts the determined characteristic(s) accordingly. This controlis preferably performed on a full time (i.e., cycle-by-cycle), fullspeed and load range basis. It may be either open loop or closed loop.Possible control schemes will now be described, it being understood thatother control schemes are possible as well.

[0068] As discussed above, the key to ignition intensity maximization isto obtain a ratio Dp/Di of <1. Dp/Di can be varied by varying pilot fuelinjection timing, Tp, pilot fuel injection duration, Dp, and/orautoignition timing, Ti. All three vary Dp/Di by varying a mixingperiod, Dm (where Dm=Di−Dp). Dm is that period between the ejection ofthe last droplets of the fuel charge from the injector and theinitiation of autoignition. Hence, the ignition intensity can bemaximized through optimization of Dm. This fact is confirmed by thegraph of FIG. 9. The curve 110 of that graph plots NO_(X) emissions vs.Dm for a Caterpillar Model 3406 engine running at 1800 RPM and full loadat various values of Tp and Dm. Dm was adjusted by varying ignitiontiming, Ti. Dp was held constant and, since Ti was nearly constant at 6°c.a. and BTDC, Di is approximately equal to Tp−6° and Dm isapproximately equal to Tp−12°.

[0069] For the data in FIG. 9, curve 110, the range of Dp/Di runs from

[0070] Dp=6° c.a.

[0071] Tp=10-40° c.a.

[0072] Di=4-34°

[0073] Dm=0-28°

[0074] Dp/Pi=1.5-0.17

[0075] Dp/Di opt=6/22 to 6/36

[0076] =0.27 to 0.17

[0077] The data used to produce FIG. 9 is reproduced below in Table 2:TABLE 2 RELATIONSHIP BETWEEN BSNOx AND Dm, BSNO_(x) Dm, ° c.a., Dm, °c.a., g/hp-h High ACT Low ACT 1.0 4 7 4.0 13 16 2.5 10 13 6.0 16 18 5.020 17 4.0 23 20 2.5 24 22 1.5 26 23 1.2 27-40 25

[0078] Actual data may vary. Curve 110′ indicates what can be expectedby using decreased ACT as a tool to adjust Di and Dp/Di. With decreasedACT or addition of EGR etc., Di is increased, providing a direct effecton increasing Dm and moving toward optimum Dp/Di and Dm.

[0079] The above is only a representative example to show trends.Optimum Dm is not constant. It varies with several factors includingengine speed, engine load, and ACT. Because the rate of fuelvaporization rises with temperature, the maximum desirable Dm variesinversely with ACT. That effect is demonstrated by FIG. 10, whichplots 1) fuel penetration and distribution percentage and 2) fuelvaporization percentage for the above-described engine running at 1800RPM and full load. Curve 120 demonstrates that, for all levels of ACT,the percentage of fuel penetration and distribution increasescontinuously up to essentially 100% after a Dm of about 25° c.a. Curve120′ indicates that the average penetration rate increases with adecrease in MAP. Fuel vaporization percentage increases more slowly atan average rate that increases with ACT (compare the low ACT curve 122(i.e., ACT≅30° C.) to the medium ACT curve 124 (i.e., ACT≅50° C.) andthe low ACT curve 126 (i.e., ACT≅70° C.). Ignition intensitymaximization occurs when 1) both the percentages of fuel penetration andvaporization and the percentage of fuel vaporization exceeds at leastabout 50%, and preferably 75%, to obtain premixed burning, and 2) thepercentage of fuel spray vaporization does not remain at 100% for morethan about 10° c.a. (misfire may occur after that point). Using theseparameters, it can be seen that optimum Dm ranges vary from 25 to 30°c.a. for low ACTs, to 20 to 25° c.a. for medium ACTs, to 18 to 23° forhigh ACTs. The data used to generate FIG. 10 I reproduced in Table 3:TABLE 3 RELATINSHIP BETWEEN Dm AND VAPORIZATION AND PENETRATION ANDDISTRIBUTION PERCENTAGES Dm, Dm, Dm, % Pene- Dm, Dm, % ° c.a., ° c.a., °c.a., tration ° c.a. ° c.a., Vapor- High Medium Low and Normal Decreasedization ACT ACT ACT Distribution MAP MAP 20 10 12 14 20 3 4 40 17 19 2140 8 10 60 20 22 24 60 11 15 80 22 24 26 80 16 20 100 23 25 27 95 20 24100 24 28

[0080] The effects of ignition intensity maximization can be appreciatedby the curves of FIG. 11. Curves 130, 132, and 134 plot instantaneousheat release (BTU/c.a.), cumulative heat release (BTU), and cylinderpressure vs. crank angle position for a Caterpillar Model 3406B enginehaving a displacement of 2.4 l/cylinder and operating at a speed of1,800 RPM and full load. Tp, Dp, and Ti are set at 18° BTDC, 6° c.a.,and 12° c.a., respectively leaving a Dm of 6° c.a. and a Dp/Di of 0.5.Due to the effects of ignition intensity maximization, the instantaneousheat release curve 130 is very steep (and, in fact, approaches vertical)during pilot fuel combustion, which occurs from about 7° to 2° BTDC.Heat is released at the rate of 0.05 BTU/° c.a. or 0.5 BTU/msec. Thishigh heat release leads to very rapid ignition of the main gaseous fuelcharge, with a peak ignition intensity of about 220 kW/l. (This estimateof heat release rate was calculated assuming that only half of theignition energy was generated by the pilot. (This percentage isadjustable by EGR and/or water injection etc.) As can be seen from curve132, cumulative heat release therefore builds very rapidly throughoutthe combustion event, reflecting effective combustion of a nearlyhomogenous mixture and low NO_(X) emissions.

[0081] Assuming for the moment that Tp and Dp are constant, Dm and,accordingly Di/Dp, can be varied by varying autoignition timing Ti. Ascan be appreciated from the curves 180, 182, 184, and 186 of FIG. 13,the effects of Di variation on mixing time will depend upon the Dp/Diobtained as a result of the Di variation and/or Dp variation. The curvesdemonstrate that Dm is much more sensitive to Di changes at low Dp/Diratios than at high Dp/Di ratios (compare curve 180 to curve 186 ).These curves also demonstrate that longer mixing times are more easilyachieved at low Dp/Di ratios, favoring the maintenance of Dp/Di ratiosof less than 0.5, and preferably less than 0.2, to permit the productionof an adequately large Dm without having to overly-retard Ti. The dataused to generate FIG. 13 is reproduced as Table 4: TABLE 4 RELATIONSHIPBETWEEN Tp AND Di Start of Injection, Tp Ignition Delay, Di, ° c.a. Deg.BTDC Low ACT Medium ACT High ACT 0 2 5 8 10 3 7 11 18 6 11 16 28 16 2126 40 28 33 38

[0082] The manner in which Ti can be varied to optimize Dm for aparticular set of engine operating characteristics requires anunderstanding of the factors affecting it. Autoignition timing isprimarily dependent on the following factors:

[0083] Engine compression ratio;

[0084] Air charge temperature (ACT);

[0085] Compression pressure (MEP);

[0086] Compression temperature;

[0087] Fuel Cetane number;

[0088] Gas fuel compression exponent, Cp/Cv;

[0089] Air/fuel ratio (Lambda);

[0090] Exhaust Gas Recirculation (EGR).

[0091] Of these factors, engine compression ratio, fuel Cetane number,and Cp/Cv are constant for a particular engine fueled by a particularfuel and without EGR or water recirculation. In addition, compressiontemperature is directly dependent on ACT, and compression pressure isdirectly dependent on manifold absolute pressure, MAP. Lambda isdependent on A) the mass of a gaseous fuel charge supplied to thecombustion chamber, B) the mass of the air charge supplied to thecombustion chamber, C) ACT, D) MAP, and E) fraction of firing cylinders,FFC, in a skipfire operation.

[0092] As discussed above, Di and, accordingly, Dm and Di/Dp can also bevaried by varying the injection timing Tp injection duration is usuallymaintained to be as short as possible and, therefore, is seldomintentionally varied. However, it may be desirable to adjust pilotquantity, injection pressure, etc., to tailor the pilot spray to beassisted in optimization of the pilot ignition event). The relationshipbetween Tp and Di varies with several factors, most notably ACT and/orEGR. This fact can be appreciated from the curves 142, 144, and 146, inFIG. 12, which plot Di vs. Tp for low ACT, medium ACT, and high ACT,respectively. These curves illustrate that, if one wishes to obtain thedesired Dm and Di/Tp by obtaining a Di of, e.g., 15° c.a., Tp will beabout 18° BTDC at a low ACT of about 30° C., 24° BTDC at a medium ACT ofabout 50° C., and 30° BTDC at a high ACT of about 70° C.

[0093] In summary, ignition intensity maximization can be achieved bymaintaining Dp/Di less than 1, preferably less than 0.5, and oftenbetween 0.1 and 0.2 or even lower. Dp/Di can be altered by adjusting Tp,Dp, and/or Di. The primary caveat is that any control of Dp/Di shouldnot result in a Dm that risks misfire. Variations in Dp/Di are oftenreflected by and dependent upon variations in Dm. Hence, pilot ignitionintensity maximization often can be thought of as optimizing Dm on afull time, full range basis. Possible control schemes for optimizing Dmwill now be detailed.

[0094] ii. Open Loop Control

[0095] Referring now to FIG. 14, one possible routine for maximizingignition intensity on a full time full, range basis is illustrated at150. The routine 150 preferably is implemented by the controller 56 ofFIG. 8 using the various sensors and control equipment illustrated inthat Figure. The routine optimizes Dp/Di by optimizing the mixingperiod, Dm. Typically, Dm will be optimized by optimizing Tp, Di, orboth. The routine 150 proceeds from START at 152 to block 154, wherevarious engine operating parameters are read, using preset values andreadings from the sensors of FIG. 8. These operating parameters mayinclude:

[0096] Governor setting or some other indication of power demand;

[0097] Engine speed (Se);

[0098] Crank shaft position (Pm);

[0099] Manifold absolute pressure (MAP);

[0100] Air charge temperature (ACT);

[0101] Exhaust gas recirculation (EGR).

[0102] The quantity of gas applied to the manifold (Q_(GAS)); and

[0103] Fuel composition;

[0104] After this data is entered, the routine 150 proceeds to block 156and initially calculates the engine operating parameters that affect Dm,including lambda, pilot fuel rail pressure, P_(RAIL), Tp, and Dp. Then,in block 158, the routine 150 determines a value of Dm required toobtain maximum ignition intensity. The optimum Dm under particularoperating conditions preferably is obtained from a look-up tablecalibrated for a full range of engine operating conditions includingspeed, load, lambda, etc.

[0105] Once the optimum Dm is determined, the routine 150 proceeds toblock 160, where a look-up table is utilized to determine the propersetting(s) of one or more operating parameters required to obtain thedetermined Dm under the prevailing engine operating conditions. Asshould be apparent from the above, the selection of the parameter(s) tobe adjusted, as well as the magnitude of adjustment, will vary basedupon several factors including the instantaneous speed and load andother, simultaneously running, routines such as a lambda optimizationroutine. As discussed above, the controlled parameter typically will bea combination of Tp, lambda, MAP, ACT and EGR if used. If Tp isconstant, or is controlled solely based on other considerations, Dm canbe adjusted by adjusting Ti. Ti can be adjusted both by adjusting theinitial air temperature (i.e., the temperature at the beginning of theinjection/combustion cycle) and by adjusting the rate of rise of the airtemperature within the combustion chamber during the compression phaseof the engine's operating cycle. In this case, the initial airtemperature can be adjusted by modifying ACT. The rate of airtemperature rise can be adjusted, e.g., by adjusting one or more ofexhaust gas recirculation (EGR), water injection, MAP, and lambda.

[0106] The look-up table contains empirically determined informationconcerning the effects of each of these parameters on Dm under variousengine operating conditions, and the controller 56 selects theparticular setting(s) required to obtain a Dm that is within anacceptable range for maximizing ignition intensity. Alternatively, Tpcan be adjusted to obtain an optimum Di and, accordingly, an optimum Dm,using data compiled, e.g., from the Tp v. Di curves of FIG. 12.

[0107] The routine then proceeds to block 162, where the controlledengine operating parameter(s) is/are adjusted as necessary to obtain thevalue of Dm determined in block 160. As a result, when a gas/air mixtureis admitted into the combustion chamber and the pilot fuel charge isinjected into the premixed charge of gas and air in block 164, thedetermined optimum Dm will be obtained, resulting in desired Dp/Di andmaximization of ignition intensity. The routine then proceeds to RETURNin block 166.

[0108] iii. Closed Loop Control

[0109] Ignition intensity could alternatively be maximized in a closedloop fashion using a measured parameter obtained, e.g., from a fast NOXsensor, a knock detector, a cylinder pressure sensor, or a flameionization detector as feedback. Fundamentally, flame ionization ispreferred as a feedback parameter because it can be relatively easilymonitored on a cycle-by-cycle basis and can provide a direct measurementof Di since Di Tp−Ti and Dm=Tp−Ti−Dp. Referring to FIG. 15, a routine200 implementing closed loop feedback control proceeds from START atblock 202 and proceeds through reading and calculation steps 202 and 204as in the open loop example of FIG. 14, except for the fact that one ormore additional values to be used as feedback, such as flame ionization,is read in block 204. Then, in block 206, the measured value of thefeedback parameter is compared to a predetermined value or range ofvalues to determine whether Dm adjustment is necessary. If the answer tothis inquiry is YES, indicating that no mixing period adjustment isrequired, the routine 200 proceeds to step 212 and controls a fueladmission, pilot fuel injection, and fuel ignition cycle withoutadjusting Dm. If, on the other hand, the answer to the inquiry of block206 is NO, indicating that the ignition delay utilized in the precedingcycle needs to be altered, the routine 200 proceeds to block 210 andalters one or more engine operating parameters to alter Dm. Just asbefore, the altered parameters could be Tp, ACT, MAP, lambda, or anycombination of them. The magnitude of the adjustment may be constant ormay be dependent upon the magnitude of the deviation between themeasured value will normally be proportional to the difference betweenthe desired Dm and the actual Dm.

[0110] The routine 200 then proceeds to block 212 as before to initiateand a pilot fuel injection, gaseous fuel/air charge admission, andignition and combustion cycle. The routine then proceeds to RETURN inblock 214.

[0111] c. Ignition Intensity Maximization Control Through PowerMaximization of Power of Pilot Ignition

[0112] Maximized ignition intensity has thus far been described in termsof optimum Dp/Di or factors relating to it such as optimum Di or optimumDm. However, it is also useful to think of maximum ignition intensity interms of the maximum instantaneous power that is generated by the pilotcharge during autoignition. Maximum instantaneous power output can beobtained by controlling injection timing, injection duration, and/orignition delay to obtain a uniform distribution of pilot fuel throughoutthe combustion chamber with an optimum size and number of droplets.

[0113] This model of ignition intensity maximization can be appreciatedthrough the use of a specific example. In a compression ignition pilot,ignited charge for an engine with 2.4 liter displacement per cylinder, a16:1 compression ratio, and a diesel pilot quantity of 2 mm³, ignitionintensity maximization occurs when the injected pilot fuel takes theform of uniformly distributed droplets of an average diameter of 50microns. If the gas/air charge is at lambda of 2.0, the projectedcombustion characteristics are as follows: TABLE 5 PROJECTED COMBUSTIONCHARACTERISTICS RESULTING FROM MAXIMIZED IGNITION INTENSITY Dropletdiameter 0.050 mm Number of droplets 30,560 Air/gas cell diameter 2 mmFlame travel 1 mm Flame speed 1 m/sec Combustion duration 1.0millisecond Ignition power in 1.0 m/sec 70 kW

[0114] In the above example, autoignition results from the instantaneouscombustion of over 30,000 droplets, each of which acts like a miniaturesparkplug. The resultant autoignition produces an instantaneous power of70 kW or about 30 kW/l, leading to extremely effective ignition of thegaseous fuel in the combustion chamber. This maximum ignition intensityis reflected by the peak on the curve 130 of FIG. 11. Other calculationshave shown that the obtainment of peak ignition intensity of over 200kW/l of displacement may be possible.

[0115] Many changes and alterations could be made to the inventionwithout departing from the spirit thereof.

[0116] For instance, while the invention has been described primarily inconjunction with an engine in which the gaseous fuel is supplied duringthe piston's intake stroke, it is equally applicable to an engine inwhich the gaseous fuel is supplied by high pressure direct injection(HPDI) during the piston's compression stroke, typically near the TDCposition of the piston. HPDI is described, e.g., in U.S. Pat. No.5,832,906 to Westport Industries, the subject matter of which isincorporated by reference.

[0117] The scope of additional changes will become apparent from theappended claims.

We claim:
 1. A method of injecting a charge of pilot fuel into acombustion chamber a compression ignition engine so as to ignite acharge of gaseous fuel in the combustion chamber, the method comprising:controlling at least one of a timing, Tp, of initiation of pilot fuelinjection, a pilot fuel injection duration, Dp, and an ignition delayperiod, Di, such that Dp/Di is <1.
 2. The method as recited in claim 1,further comprising repeating the controlling step on a full time, fullspeed and load range basis.
 3. The method as recited in claim 1, whereinthe controlling step comprises obtaining a mixing period, Dm>1° c.a.,where Dm=Di−Dp.
 4. The method as recited in claim 3, wherein thecontrolling step comprises obtaining a Dm of between 5° c.a. and 40°c.a.
 5. The method as recited in claim 3, wherein the obtaining stepcomprises altering autoignition timing, Di.
 6. The method as recited inclaim 3, wherein Di is altered by adjusting at least one of (A) atemperature, ACT, of an air charge admitted into the combustion chamber;(B) a pressure, MAP, of the air charge admitted into the combustionchamber, and (C) an air/fuel ratio, lambda, of a natural gas/air mixturein the combustion chamber.
 7. The method as recited in claim 6, whereinACT is adjusted by adjusting at least one of (A) altering a percentageof exhaust gas recirculation, EGR, from an exhaust of the engine to thecombustion chamber, (B) altering operation of at least one of 1) asupercharger, 2) a turbocharger, 3) an aftercooler, and 4) an expansionturbine located downstream of the aftercooler, (C) altering operation ofan intercooler which cools intake air being supplied to the combustionchamber, and (D) injecting water into an intake mixture.
 8. The methodas recited in claim 6, wherein MAP is adjusted by adjusting an operatingstate of a turbo air bypass valve to control a percentage of intakeairflow that bypasses the compressor output of the turbocharger of theengine.
 9. The method as recited in claim 6, wherein MAP is adjusted byadjusting a waste gate or a variable turbine nozzle of a turbocharger.10. The method as recited in claim 6, wherein lambda is adjusted byaltering at least one of A) a value of a gaseous fuel charge supplied tothe intake system or combustion chamber, B) a mass of the air chargesupplied to the combustion chamber, C) ACT, D) MAP, and E) a fraction offiring cylinders, FFC, in a skipfire operation.
 11. The method asrecited in claim 5, wherein Di is altered by adjusting exhaust gasrecirculation, EGR.
 12. The method as recited in claim 3, wherein theobtaining step comprises adjusting at least one of Tp and Dp.
 13. Themethod as recited in claim 3, wherein the obtaining step comprisesadjusting a rate of pilot fuel combustion in the combustion chamber byadjusting at least one of a size, a number, a distribution, and afraction of vaporization of pilot fuel droplets in the combustionchamber.
 14. The method as recited in claim 1, wherein the injectingstep comprises operating an electronically actuated fuel injectorcoupled to a source of a fuel that is combustible bycompression-ignition.
 15. The method as recited in claim 14, wherein theinjector comprises one which injects fuel in an expanding cloud duringat least a substantial portion of an injection event.
 16. The method asrecited in claim 15, wherein the injector includes a bottom-seatedpintle nozzle and an impingement target onto which fuel ejected from thenozzle impacts.
 17. The method as recited in claim 14, wherein theinjector comprises one of an accumulator-type hydraulic electronic unitinjector and a non-accumulator type hydraulic electronic unit injectoror any other common rail electronic fuel injection system.
 18. Themethod as recited in claim 14, wherein the pilot fuel is diesel fuel.19. The method as recited in claim 14, wherein the pilot fuel is enginelubricating oil.
 20. The method as recited in claim 19, wherein thepilot fuel charge comprises, on average, no more than about 1% of thetotal fuel charge supplied to the combustion chamber.
 21. The method asrecited in claim 1, wherein the gaseous fuel charge is admitted to thecombustion chamber during an intake stroke of the piston.
 22. The methodas recited in claim 1, wherein the gaseous fuel charge is admitted tothe combustion chamber during a compression stroke of the piston.
 23. Amethod of operating an engine having a cylinder which includes an enginehead and a piston which is reciprocateably translatable in the cylinderto define a variable-volume combustion chamber between the engine headand the piston, the method comprising the steps of: (A) performing anintake stroke of the piston; (B) performing a compression stroke of thepiston after the intake stroke; (C) admitting a gaseous fuel charge andair into the combustion chamber during one of the intake stroke and thecompression stroke; (D) injecting a pilot fuel charge into thecombustion chamber during the compression stroke, the injecting stepbeing performed by operating an electronically controlled fuel injectorcoupled to a source of fuel that is ignitable by compression ignition;(E) combusting the pilot fuel charge to ignite the gaseous fuel charge,wherein the steps of injecting the pilot fuel charge and igniting thepilot fuel charge comprise, on a cycle-by-cycle, full load and speedrange basis (1) initiating pilot fuel injection at a time, Tp, (2)continuing pilot fuel injection for a duration, Dp, and (3) igniting thepilot fuel charge by compression-ignition at an autoignition point, Ti,occurring an ignition delay interval Di after Tp; and (4) controlling atleast one of Tp, Dp, and Di to maintain Dp/Di<1.
 24. The method asrecited in claim 23, wherein the gaseous fuel charge and the air form agenerally homogenous mixture in the combustion chamber prior toinjection of the pilot fuel charge.
 25. The method as recited in claim23, wherein Dp/Di is controlled by maintaining a mixing period Dm of >1°c.a., where Dm=Di−Dp.
 26. The method as recited in claim 23, whereinDp/Di is controlled by adjusting an engine operating parametercomprising at least one of (A) a temperature, ACT, of the air chargeadmitted into the combustion chamber; (B) a pressure, MAP, of the aircharge admitted into the combustion chamber, and (C) an air/fuel ratio,lambda, of a mixture of the air charge and the gaseous fuel charge inthe combustion chamber.
 27. The method as recited in claim 26, whereinthe engine operating parameter is adjusted in an open loop withoutfeedback.
 28. The method as recited in claim 26, wherein the engineoperating parameter is adjusted in a closed loop with feedback.
 29. Themethod as recited in claim 23, wherein the controlling step comprisesactively controlling Di.
 30. The method as recited in claim 23, whereinthe controlling step comprises actively controlling Tp.
 31. The methodas recited in claim 23, wherein the controlling step comprises adjustinga rate of pilot fuel combustion in the combustion chamber by adjustingat least one of a size, a number, a distribution, and a fraction ofvaporization of pilot fuel droplets in the combustion chamber.
 32. Themethod as recited in claim 23, wherein the injector comprises one whichinjects fuel in an expanding cloud during at least a substantial portionof an injection event.
 33. The method as recited in claim 32, whereinthe injector comprises a bottom seated pintle nozzle and an impingementtarget onto which fuel ejected from the nozzle impacts.
 34. The methodas recited in claim 23, wherein the fuel source is one of diesel fueland engine lubricating oil.
 35. A method for the pilot ignition of amixture of a gaseous fuel and air in a combustion chamber of an internalcombustion engine, the method comprising: (A) injecting a pilot fuelcharge into a combustion chamber either before or after the gaseous fuelis admitted into the combustion chamber, the pilot fuel being capable ofautoignition by compression ignition; (B) igniting the pilot fuel chargeby causing a piston of the engine to undergo a compression stroke; and(C) controlling at least one of pilot fuel injection timing, pilot fuelinjection duration, and autoignition timing to generate peak power inexcess of 100 kW/l from the combustion of the pilot fuel charge in thecombustion chamber.
 36. An engine comprising: (A) a cylinder whichincludes an engine head and a piston which is reciprocateablytranslatable in said cylinder to define a variable-volume combustionchamber between said engine head and said piston; (B) a gaseous fuelsource positioned in fluid communication with said combustion chamberduring one of an intake stroke of said piston and a compression strokeof said piston; (C) an air source positioned in fluid communication withsaid combustion chamber during said intake stroke of said piston; (D) apilot fuel injector which is configured to inject a charge of pilot fuelinto said combustion chamber during the compression stroke of saidpiston; and (E) a controller which controls a device comprising at leastsaid pilot fuel injector, said gaseous fuel source, and said air sourcesuch that a timing, Tp, of initiation of pilot fuel injection, a pilotfuel injection duration, Dp, and an ignition delay period, Di, are suchthat Dp/Di is <1.
 37. The engine as recited in claim 36, wherein saidcontroller controls said device to obtain Dp/Di of <1 on acycle-by-cycle, full speed and load range basis.
 38. The engine asrecited in claim 36, wherein said controller controls said device toobtain a mixing period, Dm, of between 5° and 40°, where Dm=Di−Dp. 39.The engine as recited in claim 36, wherein said injector comprises onewhich injects fuel in an expanding cloud during at least a substantialportion of an injection event.
 40. The engine as recited in claim 39,wherein said injector comprises a bottom seated pintle nozzle and animpingement target onto which fuel ejected from said nozzle impacts. 41.The engine as recited in claim 36, wherein said fuel source is one ofdiesel fuel and engine lubricating oil.
 42. The engine as recited inclaim 40, wherein said gaseous fuel source and said air source share anair supply manifold.
 43. The engine as recited in claim 40, wherein saidgaseous fuel is injected into said intake port.
 44. The engine asrecited in claim 40, wherein said gaseous fuel is injected into saidcombustion chamber during the compression stroke of said piston.
 45. Anengine comprising: (A) a cylinder which includes an engine head and apiston which is reciprocateably translatable in said cylinder to definea variable-volume combustion chamber between said engine head and saidpiston; (B) a gaseous fuel source positioned in fluid communication withsaid combustion chamber during one of an intake stroke of said pistonand a compression stroke of said piston; (C) an air source positioned influid communication with said combustion chamber during the intakestroke of said piston; (D) a pilot fuel injector which is configured toinject a charge of pilot fuel into said combustion chamber during thecompression stroke of said piston; and (E) means for controlling adevice comprising at least one of said gaseous fuel source, said pilotfuel injector, and said air source such that a timing, Tp, of initiationof pilot fuel injection, a pilot fuel injection duration, Dp, and anignition delay period, Di, are such that Dp/Di is <1.